The present disclosure generally relates to methods for disinfecting surfaces, and in particular, methods for destroying prion molecules. More specifically, the methods use a combination of ultrasonic energy and enzyme treatment to denature and degrade the prions. The methods may be used to treat a surface, suspension, or solution contaminated with a prion or a surrogate thereof.
Many infectious agents such as bacteria, fungi, parasites, viruses, and viroids have well established methods of control that involve various forms of disinfection and sterilization (e.g. steam sterilization, dry sterilization, pasteurization, sterile filtration, treatment with ethylene oxide, glutaraldehyde, phenols or other disinfecting chemicals, radiation, etc.).
For several years, new and previously unknown pathogenic agents known as prions (“proteinaceous infectious particle”) have appeared and have been reported in scientific publications. A number of relatively similar neurological diseases have been identified both in humans and animals, that appear to be attributable to prions. These diseases are generally referred to as transmissible spongiform encephalopathies (TSEs). TSEs include Creutzfeldt-Jakob disease (CJD), variant CJD (vCJD), Kuru, Gerstmann-Straussler-Scheinker disease (GSS), and fatal familial insomnia (FFI) in humans, bovine spongiform encephalopathy (BSE) in cattle (also know as “mad cow disease”), scrapie in sheep and goats, and chronic wasting disease in elk and deer. All of these diseases attack the neurological organs of the animal or animals that are susceptible to the particular disease. They are characterized by initially long incubation times followed by a short period of neurological symptoms, including dementia and loss of coordination, and eventually death.
The structure of prions has been the subject of intense investigation and different points of view have been expressed. Some scientists believe prions are extremely small viruses, while most experts now believe that prions are actually infectious proteins without a DNA or RNA core. More particularly, infectious prions are believed to be an abnormal form of a protein commonly found in the host (i.e., a PrP or “protease-resistant protein”). The PrP gene of mammals expresses a protein which can be the soluble, non-disease, cellular form PrPC or can be an insoluble disease form PrPSc. Many lines of evidence indicate that prion diseases result from the transformation of the normal cellular form into the abnormal PrPSc form. There is no detectable difference in the amino acid sequence of the two forms. Rather, infectious prions are primarily distinguished from the cellular prion protein by their three-dimensional structure. Specifically, the cellular prion protein is predominately composed of the α-helix structure and is almost devoid of β-sheet. However PrPSc has an altered conformational form, in particular having a high level of β-sheet conformation and a large number of intra-molecular disulfide bonds, which makes PrPSc highly resistant to elimination under all but extreme conditions.
The pathogenic mechanism for prion diseases is proposed to involve a change in the normal host encoded protein. The protein undergoes a conformational change to the abnormal PrPSc form, which has the ability of self-propagation. The exact cause of this change is, at present, unknown. The abnormal form of the protein is not broken down effectively in the body and its accumulation in certain tissues (in particular neural tissue) eventually causes tissue damage, such as cell death. Once significant neural tissue damage has occurred, the clinical signs are observed.
Although prion diseases have not generally been considered to be highly contagious, they can be transmitted within a species and, under certain conditions, from one species to another. It has recently been shown that prion diseases may be transmitted via high risk tissues, including the brain, spinal cord, cerebral spinal fluids, and the eye. Iatrogenic transmission has also been reported, including transmission via dura mater grafting, corneal transplants, pericardial homografts, and human gonadotropin and human growth hormone contamination. Transmission via medical devices has also been reported. For instance, after a surgical procedure on a prion infected patient, prion containing residue may remain on the surgical instruments, particularly neurosurgical and opthalmological instruments, depth electrodes, and other devices used during surgeries in close proximity to the central nervous system. There are also concerns that groups at risk may also include veterinarians, abattoir workers, and butchers in contact with cows or beef, primarily in Europe.
There is currently much speculation about the efficacy of conventional decontamination and sterilization methods for destruction of prions. As noted above, prions are notoriously very hardy and demonstrate resistance to routine methods of decontamination and sterilization. Conventional hospital disinfectants including ethylene oxide, propriolactone, hydrogen peroxide, iodophors, peractic acid, chaotropes and phenolics have little effect on prion infectivity. In addition, infectious prions are resistant to UV irradiation, aldehyde fixation, boiling, standard gravity autoclaving at 121° C., and detergent solubilization. Although prions can be inactivated by relatively high temperatures over very long periods of time, the temperature ranges and time periods generally used to kill bacteria and inactivate the viruses are insufficient to inactivate prions. Furthermore, because prions do not contain nucleic acids, traditional sterilization methods that act by destroying or disrupting DNA or RNA are also ineffective against prions.
Some recommended methods for inactivating prions include incineration, prolonged steam autoclaving, and sodium hydroxide and sodium hypochlorite treatments at high concentrations. However, these aggressive treatments are often incompatible with expensive medical and surgical devices, particularly flexible endoscopes and other devices with plastic, brass, aluminum, or non-metallic parts. Many such devices are damaged by exposure to high temperatures, while chemical treatments, such as strong alkali, are damaging to medical device materials or surfaces in general.
Because of these limitations, prion decontamination of surgical or dental equipment is often performed only after operations on patients suspected to have CJD. Typically, standard protocols used to sterilize instruments following operations on all other patients, such as routine autoclaving, do not inactivate prions. Because of the difficulties involved in decontamination, it has also been proposed as preferable that surgical instruments used in brain surgery should be used only once. This, however, implies a disposal risk in addition to being expensive and for some instruments impractical. The extreme conditions required to destroy prions also make the cleaning of surfaces difficult, such as in a surgical or meat processing setting. Additionally, these conditions require special considerations and safety protocols be undertaken by personnel working with the instruments being cleaned.
There is thus a clear need for a cleaning process that is effective at eliminating prions, but does not use harsh conditions traditionally required for prion destruction or inactivation. Such a method could advantageously be used for routine prion decontamination of all surgical instruments to prevent cases of iatrogenic transmission of TSEs, and for decontamination of other prion contaminated surfaces, suspensions, and solutions.